German police on Tuesday arrested the wife of a Tunisian suspected jihadist already in custody over a foiled biological attack with the deadly poison ricin, prosecutors said.

The federal prosecutor's office said in a statement that the suspect identified only as German national Yasmin H., 42, had been picked up on suspicion of "planning a serious act of violent subversion".

Police commandos on June 12th stormed the Cologne apartment of Yasmin H.'s husband, the 29-year-old Tunisian Sief Allah H., and discovered "toxic substances" that turned out to be ricin.

Produced by processing castor beans, ricin is lethal in minute doses if swallowed, inhaled or injected and 6,000 times more potent than cyanide, with no known antidote.

Sief Allah H., who has been charged with possession of weapons of war and planning a serious act of violence against the state, was thought to have followed instructions on making a ricin bomb disseminated online by the Islamic State group.

Prosecutors have said they still lack concrete indications or details of an attack plan.

On Tuesday, they said Yasmin H., reportedly a convert to Islam, had organized a trip to Poland for her husband in late 2017 to buy explosive materials.

Later, they allege, she allowed him to use her online accounts to order ingredients to make ricin. They then bought a hamster to test the potency of the poison.

Yasmin H. has been considered a radical by the security authorities for some time, according to a report in Spiegel Online. The mother of six is said to have sought contact with Islamists in the past. However, she initially denied any involvement in her husband's plans.

She met her current husband, who comes from Tunisia, via Facebook, according to Spiegel. The two met for the first time in October 2015 and married immediately afterwards. Sief Allah H. worked as an intern in a post office in Tunisia. In winter 2016 he moved to Cologne to join his wife there.

Sief Allah H. had been in contact with radical Islamists and had tried twice last year, allegedly with Yasmin H.'s help, to travel via Turkey to Syria, "presumably to the 'Islamic State'" bases there.

Prosecutors say he started buying the equipment and ingredients to make ricin in mid-May - including Internet purchases of thousands of "castor seeds and an electric coffee grinder".

Police found over 3,000 castor seeds and 84 milligrams of ricin, as well as 950 grams of pyrotechnic powder, two bottles of flammable liquid, wires soldered to light bulbs and 240 metal balls.

The case comes after French authorities in May said they foiled an attack possibly involving ricin with the arrest of a 20-year-old Egyptian man.

Germany remains on high alert after several deadly attacks claimed by the IS group, including a 2016 truck rampage through a Berlin Christmas market by Tunisian asylum seeker Anis Amri that claimed 12 lives.